Individual Differences in the Experience of Pain

Neural Correlates of Inter-Individual Differences in the Subjective Experience of Pain

Some individuals say that they tolerate pain well, while others claim to be highly sensitive. Clinically, such inter-individual differences are of significant importance, but have been underappreciated due to concerns about using purely subjective criteria (i.e. pain ratings) to guide treatment decisions.

The relationship between inter-individual subjective differences and brain activity was examined in a functional magnetic resonance imaging (fMRI) investigation of 17 normal volunteers. While they had their brains scanned, a computer-controlled peltier device heated a small patch of their skin to 49°C, a temperature that most people find painful. However, different people reported very different experiences of pain. Using a 10-point visual analog scale (VAS), the least sensitive person rated his pain 1.05/10 while the most sensitive person rated her pain as 8.9/10.

People who experienced intense pain from the 49°C heat stimulus exhibited greater activation of several brain regions than people who felt that the same stimulus was only mildly painful. These regions included the primary somatosensory cortex, anterior cingulate cortex (both shown above, thresholded at z>6.5), and the prefrontal cortex. In contrast, the thalmus contralateral to stimulation exhibited generally similar activation between highly sensitive and insensitive groups of subjects.

Frequency of brain activation in highly sensitive and insensitive individuals

More people who felt the heat stimulus as intensely painful activated the primary somatosensory cortex (SI) and anterior cingulate cortex (ACC) than people who felt the heat stimulus as only mildly painful. All six of the highly sensitive individuals studied showed activation of a portion of the anterior cingulate cortex (yellow), while none of the six insensitive people activated this region.

These findings validate the subjective report and provide insight into the utility of introspection as a means of assessing a conscious experience. First-person introspection is a necessary component of the process of generating a subjective report for communication to a third-person observer. Therefore, the finding that individuals with similar patterns of activation of the primary somatosensory cortex, anterior cingulate cortex, and prefrontal cortex provided similar subjective reports of pain magnitude suggests that they can accurately capture their conscious experience via introspection. Taken together, these individual differences in pain perception and cerebral activity indicate that the most effective way of learning about a patient's pain may be to ask.

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